I have tried many different combinations of vidarg settings (setting mxcfb0, 1 and 2 to hdmi, lcd off and so on) but nothing works.
I have also tried using a DVI to DVI connector, a DVI to HDMI connector and a HDMI adapter board.

The flatflex cable between the colibri and the evaluation board is properly connected.

The linux image I'm using is Toradex's Colibri-iMX6_LXDE-Image_2.7b2-20170410.

Can you provide the output of dmesg and the content of /sys/kernel/debug/clk/clk_summary?
Have a look here.

Note that the i.MX 6 Solo and Dual Lite only have one IPU while the Dual and Quad has two.
So you only have two framebuffers mxcfb0, mxcfb1. Setting something for mxcfb2 on the cmdline will be ignored.

Here is the output of dmesg and clk_summary. I've added them in their entirety so not to miss anything. I have also removed mxcfb2:off. I already thought it shouldn't have been there but was confused because ls /dev/fb* does show fb0, fb1 and fb2.

Meanwhile I've been able to get HDMI (only on mxcfb0, mxcfb1 off) working on our own controller board using an HDMI adapter board but on the Colibri Evaluation Board I still don't get any output on the DVI.